A coiled tubing unit (“CTU”) is commonly used to perform well intervention and stimulation operations in gas and oil wells. A CTU includes a coiled tubing reel to store and transport the coiled tubing string and a specially outfitted truck to perform the installation. A common application uses coiled tubing to withdraw produced hydrocarbons from open-hole horizontal wells that have no casing. In horizontal wells with maximum reach, also referred to as “extended reach wells”, a coiled tubing string may not be able to reach total depth (TD) due to buckling, lock-up and drag/friction between the flexed tubing and the formation while running in the open-hole section of the wellbore. Installation of coiled tubing requires the truck, or rig, to remain on location which is very costly. Rigless operations cannot achieve total depth and are therefore unsuccessful with results being below expectation. This limitation can result in inefficient operations and treatment due to the inability to access sections of the well. A coiled tubing string can also become stuck in the hole, so that the only option for continuing is to cut part of the coiled tubing string and fish it out of the hole.
Prior art methods for assisting movement of a coiled tubing string under different formation conditions include applying chemical friction modifiers to the tubing and tractors both of which work only in certain operational windows. Each method also has some limitations due to the magnitude of the frictional drag forces and the total depth to be achieved. Tractors require additional drive to increase pulling of the coiled tubing string and to reach greater depths. These methods are generally not applicable in open-hole horizontal well installations.
The present invention is directed to providing solutions to the problems associated with running a coiled tubing string in extended reach horizontal open-hole wells and specifically to the problems of buckling, lock-up and frictional drag forces that slow or even prevent the completion of the coiled tubing installation and/or its removal. The terms “coiled tubing” and “coiled tubing string” are used interchangeably in the specification and claims.